Dark Screen with Normal Sound on a 14-Inch Monochrome TV
A 14-inch monochrome CRT television is brought to the workshop with normal sound but a completely dark screen. The customer says the picture disappeared suddenly after a power interruption. On the bench, the CRT heater is visibly glowing, and a faint EHT crackle can be heard when the set is switched off. With the brightness control at maximum, the screen remains dark even in a dim room. Measurements at the CRT socket give: heater 6.3 V AC, cathode 118 V DC, G1 −86 V DC, G2 310 V DC, and final anode EHT present. The B+ rail is 110 V and the horizontal output collector is also 110 V. The brightness potentiometer measures about 470 kΩ end to end, but its wiper voltage changes by only 2 V when rotated from end to end. The customer wants a cost-effective repair and asks whether the CRT is dead. The workshop has a high-voltage probe, an analog multimeter, and a CRT tester.
1. Interpret the CRT-socket measurements. Is the CRT itself the most likely cause? Explain your reasoning.
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The heater is lit and 6.3 V AC is present, so the heater circuit is working, but that alone does not prove good emission. The important clue is the cathode-to-G1 voltage: 118 V − (−86 V) = 204 V. In a small monochrome CRT, the beam is normally cut off with roughly 40–60 V of negative G1 bias relative to the cathode; 204 V is far beyond cutoff. The EHT and G2 are present, so the high-voltage and screen-supply sections are probably alive. The brightness pot reads 470 kΩ end to end, but the wiper moves only 2 V over its whole rotation. That means the G1 voltage is not being controlled; the fault is in the brightness/G1 divider, the pot’s ground return, or the CRT socket, not necessarily in the CRT. A CRT tester would confirm emission, but replacing the CRT before restoring correct G1 bias would be guesswork and wasteful. The evidence points first to the G1 control circuit.
2. Give a step-by-step isolation procedure. Include what to disconnect and the expected readings.
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- Safety first: switch off, unplug, discharge the final anode with a high-voltage probe, and work on an isolated bench. Do not remove the CRT socket while the set is running.
- With power off, measure the brightness pot from each end to the wiper. The resistance should change smoothly from near 0 Ω to about 470 kΩ. A jump or open circuit means the pot is defective.
- Trace the G1 divider resistors and their ground return. Look for an open resistor, a cracked solder joint, or a short pulling G1 toward the negative supply. Compare the measured divider ratio with the schematic.
- Check the CRT socket for leakage or carbon tracking between G1 and adjacent pins. A leakage path can clamp G1 at a fixed negative voltage.
- Temporarily inject a safe fixed bias, for example about −30 V on G1, using a high-value resistor from the existing divider. If a raster appears with the brightness control at mid-range, the CRT is probably usable.
- If G1 voltage can swing when the socket is disconnected from the CRT but not when connected, suspect CRT internal leakage or the socket. Do not run the set with the CRT socket floating for long.
- Confirm the video output cathode voltage varies with contrast; a stuck-high cathode can also cut the beam off, but here the G1 evidence is stronger.
3. Recommend the repair and state the safety checks before returning the set.
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Replace the defective brightness pot, divider resistor, or CRT socket, then restore the G1 voltage range. Set the brightness control to mid-position and adjust G2/screen and focus so that a raster appears at a comfortable brightness without retrace lines. Check that the full rotation of the brightness control gives a usable dark-to-bright range and that the picture does not bloom or defocus at high brightness.
Before returning the set, discharge and recheck the EHT, confirm the anode cap is secure and insulated, verify that no high-voltage lead is pinched, and check the mains earth and isolation. Run the receiver for at least 30 minutes and watch for arcing, smell, or overheating. Do not adjust the CRT neck components while the set is powered. If the CRT tester shows weak emission after the bias fault is repaired, tell the customer that the CRT is marginal and give a separate estimate rather than hiding the cost in the bias repair.